The present invention relates to a process for the manufacture of crystalline sodium metabisulfite. More specifically, the invention relates to an improved process for manufacturing sodium metabisulfite that is able to achieve a higher quality product and also increased yield per pass in comparison with existing processes.
Sodium metabisulfite has the following chemical formula: Na.sub.2 S.sub.2 O.sub.5. It is also known, less commonly, as sodium pyrosulfite. Sodium metabisulfite has a variety of commercial uses such as a disinfectant or bleach for fabrics and paper. It is also used as a preservative in food.
Sodium metabisulfite is typically made by crystallizing it from a solution of sodium bisulfite, NaHSO.sub.3. Under certain conditions, when a solution of sodium bisulfite is cooled, two sodium bisulfite molecules react with itself to produce a molecule of sodium metabisulfite as a crystal and water.
The sodium bisulfite can be prepared in a number of ways. Commonly, sodium bisulfite is made by reacting sulfur dioxide gas, SO.sub.2, in a solution containing sodium hydroxide, NaOH, or sodium carbonate, Na.sub.2 CO.sub.3. Also, under certain conditions, this reaction can produce sodium sulfite, Na.sub.2 SO.sub.3. However, by reacting additional sulfur dioxide gas into a solution of sodium sulfite, sodium bisulfite can be produced.
Although a variety of process schemes have been developed for producing sodium metabisulfite, these different processes are not without some disadvantages. For example, often the sodium metabisulfite product has a distinctive "rotten egg" odor from the use of sulfur dioxide gas. Also, the sodium metabisulfite product, which is in a crystalline form, often has crystals that are too small and dusty that creates handling difficulties. Sodium metabisulfite crystallizers can be quite sensitive to many disturbances that can result in a shower of unusable fines. Furthermore, because this product is a crystal, some processes have slurries of crystals being pumped from vessel to vessel. This requires more expensive equipment than would otherwise be needed if just liquids were being pumped.
To overcome this last problem, some processes complete the reactions in one vessel and crystallize the product in another vessel. However, separating the two process steps limits the optimum use of the plant equipment resulting in lower product yields.